Acyclopentadienyl complex is acoordination complex of ametal andcyclopentadienyl groups (C
5H−
5, abbreviated asCp−). Cyclopentadienyl ligands almost invariably bind to metals as apentahapto (η5-) bonding mode. The metal–cyclopentadienyl interaction is typically drawn as a single line from the metal center to the center of the Cp ring.[1][2]
Biscyclopentadienyl complexes are calledmetallocenes. A famous example of this type of complex isferrocene (FeCp2), which has many analogues for other metals, such aschromocene (CrCp2),cobaltocene (CoCp2), andnickelocene (NiCp2). When the Cp rings are mutually parallel the compound is known as asandwichcomplex. This area oforganometallic chemistry was first developed in the 1950s. Bent metallocenes are represented by compounds of the type [MCp2Lx]. Some are catalysts forethylenepolymerization.[3] Metallocenes are often thermally stable, and find use ascatalysts in various types of reactions.
Mixed-ligand Cp complexes containing Cp ligand and one or more other ligands. They are more numerous. One widely studied example is theFp dimer, (Cp2Fe2(CO)4). Monometallic compounds featuring only one Cp ring are often known ashalf sandwich compounds or as piano stool compounds, one example beingmethylcyclopentadienylmanganese tricarbonyl (CpMn(CO)3).
In the vast majority of M–Cp complexes, all 5 carbon atoms of a Cp ligand bind the metal (η5-coordination). The M–Cp bonding arises from overlap of the five molecular orbitals in the Cpπ system with the metals,p, andd orbitals; hence these complexes are referred to as π-complexes.[1]η5-coordinated Cp isisolobal to three facial carbonyls,[4] as well asscorpionate ligands and thia-crown ethers.[citation needed] Almost all of thetransition metals employ this coordination mode.[1]
In relatively rare cases, Cp binds to metals via only one carbon center. These types of interactions are described as σ-complexes because they only have aσ bond between the metal and the cyclopentadienyl group. Typical examples of this type of complex are group 14 metal complexes such as CpSiMe3. An example of both is (Cp2Fe(CO)2). It is probable thatη1-Cp complexes are intermediates in the formation ofη5-Cp complexes.
Still rarer, the Cp unit can bond to the metal via three carbons. In theseη3-Cp complexes, the bonding resembles that inallyl ligands. Such complexes, sometimes called "slipped Cp complexes", are invoked as intermediates inring slipping reactions.
The compounds are generally prepared bysalt metathesis reactions of alkali-metal cyclopentadienyl compounds with transition metal chlorides. Sodium cyclopentadienide (NaCp) andlithium cyclopentadienide are commonly used.Trimethylsilylcyclopentadienecyclopentadienylthallium (CpTl) are alternative sources.[1] For the preparation of some particularly robust complexes, e.g. nickelocene,cyclopentadiene is employed in the presence of a conventionalbase such as KOH. When only a single Cp ligand is installed, the otherligands typically carbonyl, halogen, alkyl, and hydride.
Most Cp complexes are prepared by substitution of preformed Cp complexes by replacement of halide, CO, and other simple ligands.
A pair of cyclopentadienyl ligands can be covalently linked giving rise to so-call ansa metallocenes. The angle between the two Cp rings is fixed. Rotation of the rings about the metal-centroid axis is stopped as well. A related class of derivatives give rise to theconstrained geometry complexes. In these cases, a Cp ligand as linked to a non-Cp ligand. Such complexes have been commercialized for the production of polypropylene.
Pentamethylcyclopentadiene gives rise to pentamethylcyclopentadienyl (Cp*) complexes. These ligands are more basic and more lipophilic. Replacing methyl groups with larger substituents results in cyclopentadienes that are so encumbered that pentaalkyl derivatives are no longer possible. Well-studied ligands of this type include C5R4H− (R = iso-Pr) and 1,2,4-C5R3H2− (R =tert-Bu).
Constrained geometry complexes are related to ansa-metallocenes except that one ligand is not Cp-related.
Cp metal complexes are mainly used as stoichiometric reagents in chemical research. Ferrocenium reagents are oxidants. Cobaltocene is a strong, soluble reductant.
Derivatives ofCp2TiCl2 andCp2ZrCl2 are the basis of some reagents inorganic synthesis. Upon treatment withaluminoxane, these dihalides give catalysts forolefin polymerization. Such species are calledKaminsky-type catalysts.
